1. Field of the Invention
The present invention relates to a semiconductor laser module. More particularly, the invention relates to a semiconductor laser module structure which is formed by integrating a semiconductor laser as a light source with an optical fiber as an optical waveguide and to a method of manufacturing such a semiconductor laser module.
2. Description of the Related Art
A typical semiconductor laser module is basically formed by integrating a semiconductor laser such as an Laser Diode (LD) which is an electro-optical conversion element with an optical fiber for efficiently propagating the laser light. In a semiconductor laser module, the relative position between the light emitting surface of the semiconductor laser and the incident end of the optical fiber must be accurately aligned to improve the optical coupling efficiency. Optical cavity is defined by the distance between two edge surface in ordinal semiconductor laser and the cavity length is just equal to the dimension of the semiconductor laser chip. On the other hand, it is defined by the distance between the grating formed in the core region of the optical fiber and the edge surface which is not facing to the optical fiber. In such an optical module, it will take longer time than the semiconductor laser itself from carrier injection into the cavity to generating a coherent light and deteriorate the high frequency performance of the optical module. In order to improve the performance at high frequency, it is desirable to couple the semiconductor laser directly to the optical fiber without inserting any optical members between them. Conventional semiconductor laser module mentioned above are described in, for example, U.S. Pat. No. 5,221,170.
The semiconductor laser module disclosed in that patent has a boxlike package. A ferrule is passed through one of the sidewalls of the package and fixed thereat. The grating is formed in the core region of the optical fiber protected by the ferrule. When the ferrule is thus fixed, it is precisely positioned so that a semiconductor laser and an optical fiber are efficiently coupled. In this semiconductor laser module, moreover, the ferrule is fixedly supported on its leading and rear end sides. Fine aligniment can readily be carried out by utilizing a length of the ferrule as a lever with a fixed point close to the leading end as a fulcrum.
The laser module mentioned above are subjected to a high order of alignment accuracy during the production. However, the desired performance such as optical coupling efficiency may not be always available when the laser module is installed in a system. Lowering of optical output power not relating to the deterioration of a semiconductor laser chip itself may occur. The conventional laser module has been so structured as to secure each of the members for supporting a ferrule to the wall surface of a package. Misalignment may occur because it can not avoid physical distortion by the stress induced at the packaging of the module and the temperature change in a operating even if precisely adjustment is attained at the production. A wavelength shift of the emitted light may also occur because the light reflection characteristics of the grating depends on the modulation interval of the refractive index in the grating and its absolute value. The stress applied to the package changes the modulation interval and the temperature change affects the refractive index of the grating.
A method of manufacturing the conventional semiconductor laser module has comprised the steps of securing each support member to the wall surface of the package and then fixing the ferrule to the support members. In order to pass the ferrule through these support members, however, the inner diameters of the through-holes formed in the respective support members must be greater than the outer diameter of the ferrule. Fine adjustment has been difficult because a aligning tolerance of the ferrule is too large.